As it is well known, several types of heat exchangers for condensing boilers are available on the market.
They are characterized by different yield, shape, size, material, construction technique and manufacturing costs.
Among all the already existing exchangers, the so called “plate” heat exchangers are characterized by their high compactness, heat exchange efficiency and simple construction.
The document DE 100 43 283 A1 (BOSCH) illustrates a typical plate heat exchanger used for condensing boilers.
The inlet and outlet collectors of the fumes are opposite and are close to, respectively, outlet and inlet collectors of the water, thus leading to a very efficient heat exchange called counter-current.
The fumes inlet collector has been suitably sized to house the burner, thus forming with it the actual combustion chamber of the boiler.
The document BE 764949 A1 (Riello) illustrates a solution showing several similarities to the previous document.
Both solutions have some drawbacks.
In both cases, the larger holes formed in the plates are intended to form the inlet and outlet collectors of the fumes, the larger of the two being suitably sized to house the burner, thus becoming the combustion chamber of the boiler.
Therefore, in both cases, the manufacture of the heat exchanger produces a significant amount of scraps of the sheet metal forming the plates, with a consequent cost increase.
In both cases, water and fumes run along parallel channels. Therefore, with equal flow rates, the flow of water passing within each single water channel exclusively depends on the number of plates of the heat exchanger.
However, it is well known that, to prevent the water passing through the portion of ducts facing the combustion chamber from boiling is remarkably difficult.
The thermal load per unit area transferred to that portion of ducts by fumes at about 1000° C. is so high that water immediately boils.
To avoid this problem, the water must flow at a very high speed and in large amounts; this result can not be reached if the water is distributed in parallel in single channels. In order to limit, at least partially, the waste of sheet metal caused by the fumes collector, and to decrease the amount of peripheral welding in the plates, solutions have been found in which only the plates flown through by water are closed along their outer perimeter, while the fumes channels are open along their perimeter and the whole fumes circuit is externally delimited by a casing.
Documents WO-A2-03/106909 (WORGAS) and DE-A1-10 2005 033 050 (VAILLANT) illustrate solutions in which the fumes circuit is delimited by a containment vessel.
However, even these solutions present some drawbacks.
The solution illustrated in document WO-A2-03/106909 (WORGAS) is not economical, since the circular shape given to the plates implies a large waste of the sheet metal forming the plates.
Analogously, the solution illustrated in document DE-A1-10 2005 033 050 (VAILLANT) is not economical, since the C-shape given to the plates implies a large waste of the sheet metal forming the plates.
In order to limit, at least partially, the boiling problems caused by the insufficient speed and amount of the water flowing through the channels surrounding the combustion chamber, solutions have been developed in which the speed of the water circulating in that area has been increased.
The first step was the creation, in the elements flown through by water, of some fixed paths around the combustion chamber having a limited passage section, thereby causing a consequent increase in the water speed.
Documents DE-A1-10 2005 033 050 (VAILLANT) and WO-A2-2008/107760 (GAS POINT) illustrate these solutions.
However, this method clashes with the need not to have too small passage sections in order to prevent deposits of limestone (calcium carbonate and magnesium) resulting from the precipitation of limestone (calcium bicarbonate and magnesium), which is always present in more or less relevant amounts in the water of the heating system, from rapidly clogging the pipes.